Third generation partnership project (3GPP) and Long Term Evolution (LTE) mobile telecommunication systems provide high data rate, lower latency and improved system performances. With the rapid development of “Internet of Things” (IOT) and other new user equipment (UE), the demand for supporting machine communications increases exponentially. To meet the demand of this exponential increase in communications, additional spectrum (i.e. radio frequency spectrum) is needed. The amount of licensed spectrum is limited. Therefore, communications providers need to look to unlicensed spectrum to meet the exponential increase in communication demand.
One suggested solution is to use a combination of licensed spectrum and unlicensed spectrum. This solution is referred to as “Licensed Assisted Access” or “LAA”. In such a solution, an established communication protocol such as Long Term Evolution (LTE) can be used over the licensed spectrum to provide a first communication link, and LTE can be used over the unlicensed spectrum to provide a second communication link. In LTE Release 13, LAA has been approved to enable LTE usage over unlicensed spectrum for small cells. To facilitate efficient and fair spectrum sharing, a dynamic spectrum sharing mechanism called listen-before-talk (LBT) may need to be supported based on regulation rules in each country.
Two kinds of LBT mechanisms, which are frame based equipment (FBE) and load based equipment (LBE), are proposed for spectrum sharing. For FBE, if the network is synchronized, the start timing of clear channel assessment (CCA) is the same between neighboring base stations (eNBs) and user equipments (UEs). Thus, the eNBs/UEs may not detect each other's transmission and lead to collision between the eNBs/UEs. To avoid collision, the randomness between the CCA starting time and transmission starting time is proposed. However, for synchronous network, the randomness still results in a high collision probability if the channel loading is high. On the other hand, for asynchronous network, the starting time of CCA is different between neighboring eNBs/UEs. As a result, the eNBs/UEs with late CCA starting time are always not able to grab the channel. For extended clear channel assessment (ECCA) of LBE mechanism, the original maximal size of contention window is fixed and thus LAA with LBE may be too aggressive to the Wi-Fi access points (APs) or stations (STAs). LBE with a variable size of contention window is proposed but its efficiency may not be good.
Other LBT mechanisms have also been proposed. However, the performance of LAA with LBT mechanism may not satisfy the purpose of efficient and fair spectrum sharing. Consider the scenario of LAA-WiFi coexistence: LAA with FBE may be hard to grab the channel because the starting time and time length of CCA are fixed in a FBE frame; LAA with LBE may be too aggressive to WiFi because WiFi uses the mechanism called exponential backoff. Although LBE uses contention window of a variable size, its efficiency may not be good because the adaption of the size of contention window does not consider historic channel loading information. A solution of LBT mechanism for opportunistic spectrum access is sought.